This invention relates to a proximity sensor and, more particularly, to an improved sensing head for such a sensor.
There are many instances in which it is necessary to have a device which will sense the presence or absence of an element. One example of such an instance is where a valve is used to control a positioning device, such as a hydraulic cylinder. In this instance, a sensing device may be attached to the end of the valve to sense the end position of the valve spool in order to determine the direction in which the cylinder is being actuated. For example, if the sensing device detects the presence of the spool, which indicates the cylinder is being moved in one direction, the sensing device will put out one signal, whereas if the device does not detect the presence of the spool, it puts out another signal.
It is possible to sense the end position of a valve spool through the use of a push pin type mechanical limit switch. A mechanical limit switch is suitable when it is used in a dry atmosphere and is not subjected to any extreme temperatures or pressures. However, in many applications a proximity sensor must be submerged in hydraulic fluid, which fluid may be under considerable pressure and at an elevated temperature. Additionally, the sensor may also have to detect high cycling rates, such as 2 or 3 cycles per second. It is difficult to design a push pin type of mechanical device which will operate satisfactorily under such conditions. This is because the push pin has to be sealed to prevent the intrusion of the hydraulic fluid into the switch. Furthermore, the unbalanced condition of the push pin resulting from a pressure above ambient must also be compensated for. In most push pin mechanical switches used in a wet atmosphere an overflow oil line must be connected to the inside of the switch to drain the fluid which leaks into it, since it is almost impossible to make the push pin leak proof.
In order to overcome the disadvantages of mechanical push pin type switches in applications requiring the use of a proximity sensor in a wet atmosphere, or under increased temperature and pressure, a pressure proof inductive type proximity sensor is commonly used. One type of inductive type proximity sensor currently in use consists of a transistorized oscillator circuit which is detuned by the presence of metal. When there is no metal within the range of the electromagnetic field adjacent the active face of the sensing head, the circuit oscillates. When metal is placed within the range of the electromagnetic field, the circuit develops a relatively high resistance and oscillation ceases. In some sensors, the difference in current through the circuit between the oscillating and non-oscillating conditions is monitored by a trigger amplifier and relay to give a definite switch point. That is, the switch is open or closed depending upon whether or not the circuit is oscillating. Other types of proximity sensors utilize circuitry in which a demodulator senses the changes in amplitude or frequency of the oscillating circuit.
In pressure proof proximity sensors currently in use, it is common to have a sensing head in which a ferrite core, oscillator coil and other circuit components are positioned in a cylindrical compartment in the sensor body and the compartment is closed by a thin end cap which is welded to the body or is sealed by means of an O-ring. The end cap must be relatively thin, i.e., 1.0 to 1.5 mm. to enable the core and coil assembly to get close enough to the element being sensed. A problem with this arrangement is that at a dynamic pressure of approximately 350 bars, a typical thin ceramic or plastic end cap having a diameter of 10 to 11 mm. begins to flex and ultimately either ruptures and leaks oil into the circuit components, or the flexure causes damage to the core or oscillator coil or to the wire connections. Further, the strength of the end cap decreases when the temperature of the hydraulic fluid is above 50.degree. C. Consequently, when the temperature of the fluid is increased or the dynamic pressure of the fluid is increased, the life of the sensor in terms of number of cycles is significantly reduced.
It is desirable to provide a sealing arrangement which can withstand dynamic pressures of up to or above 350 bars, tolerate hydraulic fluid which has a temperature above 50.degree. C. and has a cycle life significantly greater than has heretofore been obtainable.